The present invention relates to a golf club head, more particularly to a structure of the face portion capable of improving the durability.
In U.S. Pat. No. 6,929,566, there is disclosed a wood-type hollow metal golf club head whose face portion is formed from an alpha+beta titanium alloy Ti-6Al-4V. The face portion is decreased in the thickness to provide so called trampoline effect at impact which increases the coefficient of restitution to increase the traveling distance of the struck ball. Although the face portion as a whole is decreased in the thickness, the central region around the sweet spot is relatively thick in order to maintain the durability of the face portion.
In spite of such relatively thick central region, there is a request for further increased durability from average golfers.
In Japanese patent application publication No. 2002-165906, there is disclosed a wood-type hollow metal golf club head whose face portion is formed from a metal plate rolled in two or more different directions. This prior art teaches that if the rolled direction is one direction, the rolled plate is decreased in the resistance to bending deformation in a specific direction, and that when the rolled direction is aligned with the heel-and-toe direction of the head, the face portion is decreased in the durability. Thus, this prior art proposed to use a metal plate rolled in two or more directions and thus having less anisotropy, and also teaches that the durability of the face portion can be improved and yet it becomes not necessary to concern the orientation of the metal plate. Further, it is suggested that the metal plate is preferably formed from a beta titanium alloy by cold rolling.
In order to improve the durability of the face portion, the present inventor investigated ball hitting positions when the average golfers made miss shots. As a result, it was found that, as shown in FIG. 2 in which the ball hitting positions are mapped excluding those in the sweet area X, there is a tendency that the positions concentrate in a region At on the upper side of the horizontal line HL passing the sweet spot SS and on the toe-side of the vertical line VL passing the sweet spot SS. In particular, the positions concentrate in a region Y around the straight line K drawn between the sweet spot SS and the toe end point TP or the farthest point from the sweet spot SS. Accordingly, it is considered that the stress and strain at impact concentrate in this toe-side upper region At.
If this toe-side upper region At is partially increased in the thickness, the durability may be improved, but the trampoline effect will be biased to deteriorate the directionality of the trajectory of the ball. If the thickness of the face portion is increased in its entirety, the durability will be increased, but this defeats the original purpose.